1. Field of the Invention
The present invention relates to a compensation circuit for improving a modulation characteristic of a modulator. More particularly, it relates to a compensation circuit, which can equivalently improve a differential gain characteristic of a modulator, and the modulator and radar equipment using the same.
2. Description of the Related Art
In radio apparatuses for transmitting FM signals or FM-CM (Continuous Wave) radars, it is required that a differential gain (DG) of an FM modulator be small. To fulfill the requirement, a conventional circuit has obtained a suitable DG by controlling constants of high-frequency circuits of the modulator as a first example, or the circuit has equivalently improved the DG by modifying a base band waveform, which is added to the modulator in a compensation circuit as a second example.
However, it has become more difficult to control the constants if a oscillating frequency in the modulator is within a micro wave band or millimeter wave band in the above-described first example.
The above-described second example will be explained in accordance with FIGS. 21A through 21C. Throughout FIGS. 21A through 21C, a compensation circuit 1 modifies a base band signal. FIG. 21A shows the compensation circuit and its compensation characteristic.
FIG. 21B shows a voltage-controlled oscillator 2, in which an output frequency f is varied according to control electric voltage V.sub.T, and its input/output characteristic. Hereinafter, a percentage of variation of modulation sensitivity, i.e., DG=(kV.sub.0 -kV.sub.1 / kV.sub.0) is referred to as a differential gain in the input/output characteristic of the voltage-controlled oscillator 2. kV.sub.0 and kV.sub.1 show modulation sensitivities.
If the voltage-controlled oscillator 2 has an ideal characteristic, the relation between the control voltage V.sub.T and the output frequency f has a linear characteristic, and the differential gain becomes 0 .
In the case where the input/output characteristic of the voltage-controlled oscillator 2 shows a non-linearity as shown in FIGS. 21B, a characteristic of the compensation circuit 1 should be reversed to the input/output characteristic of the voltage-controlled oscillator 2, as shown in FIGS. 21A. Further, the compensation circuit 1 is connected to the voltage-controlled oscillator 2 in series, as shown in FIG. 21C. Therefore, the relation between the input V.sub.in and the output frequency f has a linear characteristic after composing the input/output characteristic of the voltage-controlled oscillator 2 and the characteristic of the compensation circuit 1, so that the differential gain can be improved.
Accordingly, if the characteristic of the compensation circuit 1 is reversed to the input/output characteristic of the voltage-controlled oscillator 2, it is possible to obtain the characteristic having a suitable linearity after composing. FIGS. 22 and 24 show structures of the conventional compensation circuit 1. Further, FIGS. 23 and 25 show characteristics of the structures, respectively.
The compensation circuit shown in FIG. 22 is disclosed in Japanese Unexamined Patent Publication No. 20004/1982. The compensation circuit is composed of a diode D.sub.1 inserted between input and output sides in a forward direction, a variable resistor R.sub.v connected to the diode D.sub.1 in parallel, and a resistor R connected to the output side in parallel.
A bending line characteristic obtained in this circuit is employed to bring the characteristic close to an inverse characteristic of the input/output characteristic of the voltage-controlled oscillator 2. FIG. 23 shows a characteristic which is close to the inverse characteristic of the input/output characteristic. This circuit, has a drawback drawback that a bending point is varied as B.sub.P1, to B.sub.p3, as shown in the diagram, when a desired bending line characteristic is obtained, according to the size of the resistor R.
FIG. 24 shows a circuit which is disclosed in Japanese Unexamined Patent Publication No. 224706/1986. The circuit is composed of an operational amplifier OPA having a feedback resistor R.sub.f and a FET connected to an input side of the feedback resistor R.sub.f.
FIG. 25 shows a V.sub.g -I.sub.d characteristic of a gate voltage V.sub.g and a drain current I.sub.d of the FET. The circuit shown in FIG. 24 compensates the input/output characteristic of the voltage-controlled oscillator 2 by employing a non-linearity section of the V.sub.g -I.sub.d characteristic.
However, in the V.sub.g -I.sub.d characteristic shown in FIG. 25, a percentage of non-linearity in an area (II) is larger than that in an area (I). Therefore, the V.sub.g -I.sub.d characteristic in the area (II) is largely changed by the input V.sub.in, and the characteristic does not change in the area (I).
Accordingly, it is possible to compensate the characteristic in the area (II) in which the control voltage is small for the output of the voltage-controlled oscillator 2. However, there is a drawback that it is not possible to compensate the characteristic for the area (I) in which the control voltage is large.